Impact modifiers used in order to improve the impact resistance of vinyl chloride resins include methylmethacrylate butadiene styrene (MBS) resins, chloroethylene (CPE) resins, acrylic resins, etc., where it is necessary to use rubber polymers having a low glass transition temperature (Tg) to the maximum in order to improve the impact resistant strength of thermoplastic resins.
Among them, in butadiene styrene resins, the polybutadiene rubber is more superior to polybutylacrylate, which is the rubber component of acrylic impact modifiers, in view of its impact resistance since the glass transition temperature (Tg) of the polybutadiene rubber is about −80° C., which is lower than the glass transition temperature (Tg) of polybutylacrylate, i.e., about −50° C.
However, the polybutadiene rubber is problematic in that it is thermally unstable since there are unsaturated double bonds. Therefore, acrylic resins having no unsaturated double bonds are used widely as the impact modifiers of outdoor plastic products that are exposed to the day light greatly since they have superior weatherability.
In other words, for the products requiring for impact resistance and waterproofing like window frames, etc., polymers having the core-shell structure in which methacrylate polymers that are compatible with vinyl chloride resins are grafted to the rubber core composed of alkyl acrylate are used mainly. However, the above polymers are problematic in that their impact-resistance improvement effects are insufficient since their glass transition temperature is higher than that of butadiene rubber.
In the meantime, polyorganosiloxanes, also known as polydimethylsiloxanes, are very effective polymers for the improvement of impact resistance since their glass transition temperature (Tg) is around −120° C., and therefore, it is expected to improve weatherability as well as impact resistance by using acryl-silicone hybrid rubber graft copolymer particles containing the silicone component.
The factors determining the physical properties of acryl-silicone hybrid impact modifiers having the core-shell structure include the rubber content of impact modifiers, size of rubber particles, distance between rubber particles, swelling index for the solvent, degree of bonding among impact modifier particles dispersed according to processing and matrix, etc. Particularly, bonding between an impact modifier and matrix is determined according to the grafting efficiency of the shell with respect to the core rubber of the impact modifier.
Acryl-silicone hybrid impact modifiers in order to improve the impact resistance of vinyl chloride resins are manufactured through the emulsion polymerization method usually, which include the following methods:
The first method is, as in the U.S. Pat. No. 5,132,359, a method of completing a core-shell structure by polymerizing polydimethylsiloxane rubber seeds firstly; charging butylacrylate monomers as the rubber core component in order to grow rubber particles, subsequently adding shell component monomers to cover the core surface finally.
The second method is, as in the U.S. Pat. No. 6,153,694, is a method of microagglomeration by polymerizing separately each of the polydimethylsiloxane rubber latex of less than 100 nm and the polybutylacrylate latex; growing them to particles of a desired size through agglomeration; and forming a core-shell structure by forming encapsulated shells finally.
Usually, shell polymerization is done through graft polymerization of methylmethacrylate monomers that may be compatible with vinyl chloride resins on the surface of the core, or through graft polymerization by adding a small amount of monomers having two or more functional groups. Particularly, methylmethacrylate assumes the role of improving the agglomeration characteristics of the latex since it is highly compatible to the matrix and has a relatively high glass transition temperature.
However, the above method is still disadvantageous in that it fails to have high impact-resistance improving effects and high-gloss characteristic but has a long polymerization time of organosiloxane polymers. Accordingly, in reality, vinyl chloride resins overcoming the above-described problems and having superior impact resistance, weatherability, and high-gloss characteristic simultaneously are demanded continuously.